1. Field of the Invention
The present invention relates to a toner, a two-component developer containing the toner, and an image forming apparatus using the toner.
2. Description of the Related Art
In recent years, image forming apparatuses have been required to provide a higher quality image, and development has been made on a toner excellent in, for example, heat resistance storageability, transferability, flowability, filming property and chargeability.
In order to improve a toner in heat resistance storageability, a toner having a core-shell structure is proposed which has, on the toner surface, a shell layer containing a resin different from a binder resin (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2006-267950). In this proposal, however, a pigment is not successfully dispersed in the shell layer and localized in the surface. As a result, this toner is poor in image quality due to its poor transferability and flowability.
In order to improve the above toner in transferability and flowability, there is proposed a toner to which an external additive containing inorganic particles has been added (see, for example, JP-A Nos. 2005-173480 and 2010-128216). According to these proposals, the external additive acts as a spacer on the toner surface, preventing adhesion between toner particles and formation of aggregates of toner particles during transportation. As a result, the toner is improved in transferability and flowability, achieving reduction of abnormal images resulting from degradation of the toner. In these proposals, however, the inorganic particles are excessively added to the toner and thus are easily exfoliated. The exfoliated inorganic particles accelerate abrasion of a cleaning blade and cause filming, raising a problem that abnormal images are formed due to impaired chargeability.
In order to improve the above toner in filming property and chargeability, there is proposed a toner to which silica having a relatively broad particle size distribution have been added as an external additive (see, for example, JP-A Nos. 11-174731 and 2010-243664). According to these proposals, an external additive having a relatively broad particle size distribution can provide a toner with a wide range of chargeability depending on the particle size distribution of the toner. In terms of an ability to impart chargeability to the toner, the silica is superior to alumina. The silica is not limited to silica obtained by a sol-gel method (i.e., sol-gel silica), and use of dry silica is recommended since the dry silica can provide a toner with a wide range of chargeability depending on the particle size distribution of the toner. The toners according to these proposals, however, use the dry silica only, and involve a problem of being poor in heat resistance storageability. When the toner is made to have a sharp particle size distribution, the external additive having a broad particle size distribution does not have to be used. Even an external additive having a sharp particle size distribution can provide uniformly chargeable toner particles. The dry silica has a non-spherical shape and contacts with each toner particle at a plurality of contact points. Thus, the dry silica is more difficult to roll on the toner than a spherical silica does thereon, so that the toner tends to be impaired in flowability.
Therefore, at present, strong demand has arisen for rapid development of a toner that is satisfactory in all of heat resistance storageability, transferability, flowability, filming property and chargeability as well as is excellent in image quality.